In recent years, in the field of optical communication, speed and capacity of an optical transmission system are increasing. As a core technology of the increases, a wavelength division multiplexing (WDM) system, in which light is multiplexed and transmitted with one optical fiber, is popular. Wavelength division multiplexing is hereinafter called “WDM”. In this WDM system, in order to perform stable operation, as an insurance against an unexpected stop of a signal light source, it is required to secure an auxiliary signal light source having the same oscillation wavelength. Therefore, a maintenance cost is increased. In order to suppress this maintenance cost, there is an increased demand for a wavelength tunable light source with which a plurality of laser beams can be output from one signal light source.
As a representative wavelength tunable light source, there has been proposed a system in which a temperature of a semiconductor laser is changed to make an oscillation wavelength tunable. A tunable width of the oscillation wavelength of the semiconductor laser employing such system is determined by an operating temperature range, and is at most from about 2 nm to about 3 nm. Therefore, in order to increase a wavelength tunable width, a structure including a plurality of semiconductor lasers is often used.
It is required for the wavelength tunable light source to have a stable wavelength of an optical signal for a long period of time. Therefore, there has been developed alight source having a function of a wavelength monitor configured to monitor emitted light from a semiconductor laser. As a representative related technology regarding the wavelength monitor of the wavelength tunable light source, there is proposed the structure in which a portion of light is extracted, by a beam splitter or the like, from light emitted from a front surface of the semiconductor laser to be output to an optical fiber, and the extracted light is allowed to enter an optical filter formed of an etalon (see Patent Literature 1, for example).
It is also proposed, in a wavelength monitor configured to monitor wavelengths of laser beams emitted from two or more semiconductor lasers, which are formed in parallel on a semiconductor substrate, to dispose emitted light positions of the semiconductor lasers at unequal intervals. In the wavelength monitor, emitted light emitted from the semiconductor lasers in a direction of a rear surface of the semiconductor substrate is allowed to enter an optical filter formed of an etalon (see Patent Literature 2, for example).
There is further proposed, in order to relax amounting accuracy of an optical filter, the structure in which two laser beams are allowed to enter an optical filter formed of an etalon at symmetrically positive and negative angles (see Patent Literature 3, for example).
Citation List
Patent Literature
[PTL 1] JP 2002-185074 A
[PTL 2] JP 2012-129259 A
[PTL 3] JP 2015-35553 A